Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and then automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
A number of approaches have been developed to control the torque relayed between the engine and the wheels of the vehicle during idle-stops and subsequent restarts. For example, the torque conveyed may be reduced by engaging one or more transmission gears, tying up the transmission to a transmission case, applying an alternator load on the engine, shifting transmission gears, etc.
However, the inventors have recognized a potential issue with such approaches. While the various approaches may improve torque mitigation during an engine restart, they may also result in reduced driveline torque. Specifically, there may be a reduction in the amount of torque transmitted from the engine to the driving wheels during the restart. Consequently, in response to an accelerator pedal tip-in by the vehicle operator, a desirable fast launch may not be achieved. As such, this may be perceived by the vehicle operator as a sluggish vehicle performance. Additionally, NVH issues may arise. Overall, the quality of the engine restart may be degraded.
In one example, the above issue may be at least partly addressed by a method of operating a vehicle engine coupled to a transmission. In one embodiment, the method comprises, during an engine restart from idle-stop, when operator torque demand is lower than a threshold, reducing an engine torque conveyed to vehicle wheels over an interval of the engine restart. Then, in response to the operator torque demand being higher than the threshold, decreasing the engine torque reduction. In this way, torque reduction can be advantageously used during an engine restart to reduce vehicle lurch, while the torque reduction can be lowered to return driveline torque and expedite vehicle launch.
In one example, torque reduction during the engine restart may be achieved by restarting the engine with the transmission upshifted to a higher gear. The transmission may be upshifted at the onset of the restart or during the preceding idle-stop. In response to a torque demand being higher than a threshold, such as during an accelerator pedal tip-in by the vehicle operator, the driveline torque may be substantially immediately restored by downshifting the transmission to a lower gear, even if the engine speed is below a threshold engine speed. In another example, the engine shaft output torque may be reduced during the engine restart via adjustments to an engine torque (for example, by applying an alternator load on the engine, by skipping one or more combustion events, by retarding spark timing, etc.) and/or a transmission torque (for example, by engaging one or more transmission clutches, by varying an amount of transmission clutch slippage, by tying up the transmission, by upshifting the transmission, etc.). In response to the tip-in, based on the amount of driveline torque requested, the transmission may be downshifted and/or the engine shaft output torque reduction may be decreased (e.g., by lowering the engine torque reduction and/or the transmission torque reduction). In comparison, if the torque demand is lower than the threshold, the transmission may be downshifted only after the engine speed has reached a threshold engine speed (e.g., an idling speed).
In this way, torque reduction may be used over an interval of the engine restart to reduce the effects of an engine restart torque spike. By decreasing (e.g., stopping) the torque reduction based on the driveline torque requested, the vehicle may be better transitioned through the engine restart torque spike while also reducing the time required to provide the requested driveline torque. Additionally, the restart torque spike can be blended with the vehicle acceleration to improve the quality of the engine restart.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.